Project Summary/Abstract Extraordinary advances are being made in our understanding of the immune system, with discovery of functionally specialized cell types. However, new data are beginning to reveal that the process by which hematopoietic stem cells (HSC) develop is more complex and less rigidly controlled than generally believed. Textbooks have long depicted simple relationships between HSC and B lymphocytes, but it now appears that there is not just one branch point where progenitors lose the option of becoming B cells and are directed to other fates. Rather, dedication and firm commitment to the B lineage is a gradual process and can be dramatically altered in response to environmental cues. There is still much to learn about endogenous cues such as those provided by cytokines, hormones and cell adhesion molecules. In addition, we have new observations suggesting that retinoids can accelerate the process of lymphocyte formation and their effects on the bone marrow merit further study. For example, these widely prescribed drugs hold promise as agents for boosting recovery of the immune system. We are combining special knock-in mice with cell sorting, cell culture and transplantation techniques to systematically investigate these and other questions. Our lab discovered that Toll-like receptors (TLR) are expressed on hematopoietic cells, allowing these cells to recognize bacterial/viral products and several important outcomes of this interaction have already been identified. For example, TLR ligands stimulate stem cells to enter cycle and begin differentiating, while myeloid restricted cells complete their maturation. The ligands arrest B cell production and cause lymphocyte progenitors to generate several types of dendritic cells. While these new mechanisms may have survival value, we believe there are also circumstances where stem and progenitor cells need to be protected from such substances. Our lab is using defined culture conditions to study mechanisms associated with an apparent re-programming of lymphoid progenitors. In addition, we are tracking the same phenomena in HSV infected mice. Project Narrative: Our bone marrow must constantly make new blood cells, including cells responsible for antibody formation and defense against infections. Patients are vulnerable to infections following chemotherapy or transplantation, but our data suggest that it may be possible to accelerate recovery of the immune system. Basic understanding of early stages of immune system development may point the way to improved healing of damaged tissues as well as better treatments for cancer and autoimmune diseases.